polyesters are polycondensates. So all that needs to be done is to provide the polyester with the abstracted group (water) and give it sufficient thermal energy.

In simple words, make sure that the polyester is wet, then heat it by excluding oxygen, e.g. hot press it in melt state. I did that a long time ago with polylactides and 210°C without proper drying reduced the molar mass from 900 kDa to ca. 120 kDa.

The key process parameters should be

- moisture content

- temperature

- duration of heat treatment

however, you need to be aware of one potential problem. Polyesters tend to loose their mechanical strength quickly when reducing the molar mass. In my case, I had a rather tough PLA when not reducing the molar mass (it withstood quite a significant bending deformation (too big to break it in a 3 point bending geometry), while the low molecular variation might break when only dropping the sample by 30 cm on a hard surface.

Thank you very much Prof. Dr. Stadler. This information will be very useful ! I think a decrease in mechanical strength won't be a big problem, once we are about to use the reduced MW polymers in a reaction of copolymerization and the following product will be used as a compatibilizer. Again, thank you for the information and time.

I'd recommend recirculating the melted polyester in a single screw or double screw extruder.  The heat (Tm +20) and shearing action of the extruder will gradually break down the molecular weight through beta-scission and autooxidation--if antioxidants are present, they will eventually be consumed. By converting screw torque into viscosity values, you can estimate the rate of molecular weight degradation.  As Florian said above, a little moisture will accelerate MW degradation by ester hydrolysis. You may be able to do your reactive copolymerization directly in the extruder if it will dissolve into the melt.

to reduce the heat temperature

to elavate the vacuum degree

to decrease the polymerization time